The present invention relates to a fluorescent aqueous marking liquid with a content of cumarin C.J.Basic Yellow 40 dye and, as necessary, an additional dye, and with a content of synthetic resin and, as necessary, a polyfunctional alcohol, such as glycol or its oligimers.
Aqueous writing and marking fluids to which a synthetic resin, e.g. polyacrylate, is added for assisting the fluorescent action, are described in German Open Patent Application 25 07 064. Different products have been mentioned as dyes, among others cumarin C.J.Basic Yellow 40 dye, which is marketed by the Firm, Ciba-Geigy under the name Maxilonbrillantflavin 10 GFF. Essential to the products described in this disclosure, the dyes react as a whole as a base, which is required to keep the above-mentioned polyacrylic resin in solution.
A marking liquid is described in the German disclosure 23 15 680 which contains hydroxypyrenetrisulfonic acid as a fluorescent dye. That the marking liquid with the fluorescent dye should be adjusted to a pH value which is in the alkaline region and advantageously to a value above 8.5 is basic to the above-named disclosure. The fluorescent effect of the above-named fluorescent dye develops completely in this pH region.
Although advantageously the alkaline pH-value may be adjusted in relation to the fluorescent effect and the intensity of the color regions, the alkalinity of the marking liquid can have a disadvantageous effect on the marking liquid. Particularly when the marking liquid is used on carbon paper, which contains triphenylmethane dye--especially in microencapsulated form--, the alkaline reaction of the liquid can lead to conversion of these compounds into the noncolored Leukoform and the writing can be extinguished because of that.